First Principle Study On Stability And Electronic Structure Of Doped ZnO

ZnO is a direct wide-band gap (3.37 eV) II-IV compound semiconductor with large exciton binding energy (60 meV) at room temperature. More attention has been paid to ZnO-based films due to their potential applications in optoelectronic devices in blue and ultraviolet spectra of light. A crucial step in designing modern optoelectronic device is the realization of band-gap engineering to create barrier layers and quantum wells in device heterostructures. In order to realize such optoelectronic devices, two important requirements should be satisfied: one is p-type doping of ZnO and the other is modulation of the band gap. This work is focus on the stability and electronic structure of the ZnO-based materials. Using first principle calculation, Cd, Be and Mg-doped ZnO are studied for band gap engineering. For p type doping, Cu, Ag-doped and Li-N, Be-N co-doped ZnO are studied. The results are summarized as follow:1. Analysis of the band structures, density of states (DOS) and partial density of states (PDOS) of Cd_xZn_1-xO shows that the valence band maximum (VBM) is determined by O-2p states and the conduction band minimum (CBM) is occupied by the hybrid Cd-5s and Zn-4s orbital. With increasing Cd-doping concentrations, the energies of CBM decrease and the energies of VBM increase, hence leading to narrowing of the band gap. Furthermore, we find that the reduction of band gap due to Cd-doping can be attributed to the tensile strain, which is about 20-30% in the reduction of the band gap. We also study the interaction between Cd and native defects. It is found that the Cd incorporation can lower the formation energy of V_o defect and results in the production of Cd_Zn-V_o complex when the Cd dopant is close to V_o. The high transition level, however, suggests that the Cd_Zn-V_o complex cannot be the source resulting in the increase of the concentration of n-type carriers due to Cd doping. We study a more complicated structure Zn_i-Cd_Zn-V_o and suggest that the increase of n-type carriers caused by Cd doping might result from a structure similar to Zn_i-Cd_Zn-V_o, such as Zn_i-2Cd_Zn-2V_o and Zn_i-3Cd_Zn-3V_o.2. The band structure, DOS, and orbital energy level of Be-doped and Mg-doped ZnO are investigated. The results show that the absence of d electrons in Be and Mg can weaken p-d repulsion, hence resulting in the decrease of VBM in the doped ZnO. The energy levels of CBM are found increasing in the doped ZnO. The increase can be attributed to the higher orbital energy levels of s electrons in Be and Mg atoms. In addition, Be-doping can cause compressive strain, causing the increase of band gap about 20%-30%. The formation enthalpy of alloy is also discussed. Mg-doped ZnO is more stable than Be-doped ZnO due to the little difference in radii between Mg and Zn ions.3. The electronic structures of the Cu_xZn_1-xO and Ag_xZn_1-xO wurtize alloys are calculated. Although Cu and Ag have the similar ion radius with Mg and Cd, respectively, the formation enthalpy of Cu-doped and Ag-doped ZnO is found to be higher than Mg-doped and Cd-doped ZnO. The results of electronic structures show that Cu-doped and Ag-doped ZnO have narrower band gap than ZnO, which can be attributed to the extension of impurity levels induced by Cu-3d and Ag-4d. Because the impurity levels is close to the VBM of ZnO, heavy doping results in the impurity levels extending to the VBM and merging into one band. Therefore, the reduction of band gap appears sharply at a concentration of Cu or Ag dopant. Thereafter, the band gap varies slowly, which can be attributed to the influence of Cu-4s or Ag-5s electrons on the CBM energy.4. Formation energy, transition energy and binding energy of defects in Li, N and Be, N co-doped p-type ZnO are calculated. For Li, N co-doped ZnO, it is found that Li_Zn is the cause of p-type conductivity in O-rich condition. The p-type conductivity is dominated by the Li_i-Li_Zn-No in Zn-rich condition because it has relatively small formation energy and transition energy and lowest binding energy. For Be, N co-doped ZnO, nBe_Zn-N_o and Be_i-nN_o are discussed. In O-rich condition, acceptor 4Be_Zn-N_o may be the source of p-type conductivity. In Zn-rich condition, both of 4Be_Zn-N_o and Be_i-5N_o are found to be the most possible candidates to provide the p-type conductivity.5. First-principles ultrasoft pseudopotential method is applied to study HX ZnO, which has a novel graphitelike hexagonal structure transformed from wurtzite phase under tensilestress along [01 (?) 0] direction or compressive stress along [0001] direction. The electronic structure and optical properties, including dielectric function, reflectivity and absorption coefficient, of HX ZnO are calculated and compared with those of WZ ZnO under the given uniaxial stress. It is found that HX ZnO is an indirect semiconductor, being different from WZ ZnO. HX ZnO has a dielectric response different from WZ ZnO at ambient conditions or under the given uniaxial stress, especially in the case of E//c. Similar variation is also observed in the reflectivity and absorption coefficient. The variation in the optical properties is attributed to the additional Zn-0 bond along c-axis HX ZnO. In addition, the electronic structure and optical properties of LY ZnO are reported.